FIG. 1 shows apparatus for producing a color image signal that is exemplary of the prior art. For example, see "CCD Digital Color Camera" by Nagumo et al, IEEE Transactions on Consumer Electronics, August, 1980 Vol. CE-26, No. 3, page 339. An image, formed by lens 10, is separated into a green (luminance) component and a red/blue (chrominance) component by a dichroic mirror 12 positioned between a pair of prism blocks 14 and 16. The green image component is passed by dichroic mirror 12 and falls on a solid state image sensor 18, for example a charge coupled device (CCD) image sensor having a regular array of image sensing elements. The output of solid state image sensor 18 is amplified by an amplifier 17 and sampled by a sample and hold circuit 19 to produce a signal G.sub.S representing the green component of the image.
The output of the CCD image sensor 18, from amplifier 28, is a time varying waveform including undesirable clock and reset noise signatures, and a momentary level during each sample readout period representing the desired image sample value. The sample and hold circuit is clocked at the proper moment by clock signal .phi..sub.G to capture and hold this signal level until the next sample is read out. The red/blue component of the image is reflected by dichroic mirror 12 and prism block 14 and falls on a solid state image sensor 20.
The solid state image sensor 20 is provided with a regular array of first and second colored filter elements (for example, a striped filter 22 as partially shown in FIG. 2) having alternate red and blue filter elements to separate the red/blue component of the image into a red chrominance component and blue chrominance component. The word "regular" means that the image sensing elements and the color filter elements occur at uniform spacings in the array. The filter array is registered on the solid state image sensor 20 so that the filter elements are aligned with the columns of image sensing elements on the image sensor. The word "registered" means that the filter array is physically attached to the surface of the sensor, or that the image of the filter array is optically projected onto the surface of the image sensor.
The output of solid state image sensor 20 is amplified by amplifier 28 and sampled by sample and hold circuits 30 and 32 clocked by clock signals .phi..sub.R and .phi..sub.B respectively, to provide a signal R.sub.S representing the red chrominance component of the image, and a signal B.sub.S representing the blue chrominance component of the image. The sample and hold circuits 30 and 32 function not only to recover the image sample signals from the noisy output of the image sensor, but also to demultiplex the separate color component signals from the output of the image sensor.
For display on an R, G, B color monitor, the sampld analog signal components R.sub.S, B.sub.S and G.sub.S of the color signal may be low pass filtered and applied directly to the color monitor. For display on a standard color television receiver, the sampled analog signal components R.sub.S, B.sub.S and G.sub.S of the sampled color image signal produced by the camera are processed in an NTSC encoder. The green component G.sub.S, which has twice the sample rate of the blue and red components (B.sub.S and R.sub.S) is employed to produce the high resolution luminance component (Y) of the NTSC signal, and a subsampled version (e.g. every other sample) of the green signal component is employed with the red and blue components of the image signal to produce the chrominance (I and Q) components of the NTSC signal.
The response of the prior art solid state image sensor 20, with color filter array 22, to a component of scene detail, for example a vertical white stripe on a dark background, is illustrated schematically in FIG. 3. The line labeled W in FIG. 3 represents the brightness profile of a vertical white strip falling across a horizontal line of image sensing elements in the solid state image sensor 20. The vertical columns labeled "RBRB . . . " in FIG. 3 represent the red and blue filter elements registered on alternate image sensing elements in the solid state image sensor. The line labeled R.sub.S in FIG. 3 is the signal representing the sampled red chrominance component of the white striped image, and the line labeled B.sub.S is the signal representing the sampled blue chrominance component of the image. As can be seen in FIG. 3, the R.sub.S and B.sub.S signals have different amplitudes in areas where the brightness of the image is changing, i.e. at the edges of the white stripe. The areas where the signals differ have been indicated by the cross-hatched regions labeled 34 in FIG. 3. When the color image signal is displayed, the differences in the red and blue chrominance components of the signal at the edges of the stripe causes undesirable color fringing to appear in the image.
The object of the present invention is to provide apparatus for producing a color image signal having first and second color components, of the type having an image sensor with a regular array of first and second colored filter elements registered on a regular array of image sensing elements, that produces a color image signal free from such color fringing.